1. Field of the Invention
The present invention relates to a laser printer and particularly to a laser printer capable of automatically controlling an image density.
2. Description of the Prior Art
In a laser printer, a latent image is formed on a photoconductor drum by using a laser beam deflected by a polygon mirror rotating at high speed, and the image is caused to be visible by development of toner and then it is transferred onto plain paper and fixed. Since the laser beam can be modulated at high speed in a laser printer, the laser printer achieves printing of letters or graphics of high quality (high density) at high speed. Accordingly, laser printers are widely utilized as output apparatus of various data processing systems or image processing systems using computers.
The above described printing process of a laser printer is similar to an electrophotographic process and a density of an image printed finally on plain paper is changed dependent on an amount of toner. Consequently, in order to maintain a density of an image in the most suitable state, it is necessary to control the amount of toner.
On the other hand, in the technical field of copying apparatus using an electrophotographic process, the U.S. Pat. Nos. 4,272,182, 4,239,372 or 4,416,535 for example proposes a density control system in which an electrostatic latent image corresponding to a mark of a predetermined density is formed and the density of the toner image based on the electrostatic latent image is detected to control supply of toner based on the detected density. The mark is provided on a lower surface of an end of a document table of glass and an image of the mark is projected on a photoconductor by means of an optical system, whereby the electrostatic latent image corresponding to the mark is formed.
If the above described density control system is applied to a laser printer, an electrostatic latent image (i.e. a reference image) corresponding to a mark is formed by the scanning of the laser printer. However, the reference image should be formed outside an image area and in a fixed position corresponding to a position of a toner density sensor. Positioning of an image area is controlled usually by using a timer or a counter based on a signal from a sensor for detecting a start position of scanning of the laser beam (this sensor being called a start-of-scan sensor). In the case of applying the above described density control system to a laser printer, it is necessary to provide a timer or a counter for a reference image, other than a timer or a counter for an image area and accordingly construction of the printer is complicated and the manufacturing cost thereof becomes high.